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Unpaid losses and loss adjustment expenses - Narrative (Detail) - Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] - USD ($) | 9 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Losses and loss adjustment expenses incurred, current accident year | $ 43,124,000,000 | $ 43,166,000,000
Losses and loss adjustment expenses incurred, prior accident years | $ (1,182,000,000) | $ (992,000,000)
Prior years' losses incurred as a percentage of net liabilities | 1.10% | 0.90%
Primary Insurance Businesses [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Losses and loss adjustment expenses incurred, prior accident years | $ (284,000,000) | $ 222,000,000
Property and Casualty Reinsurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Losses and loss adjustment expenses incurred, prior accident years | (898,000,000) | (1,200,000,000)
Catastrophe [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Minimum losses per event each accident year | 150,000,000 |
Losses and loss adjustment expenses incurred, current accident year | $ 1,100,000,000 | $ 715,000,000

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- Definition Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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